The present invention is directed to storage cells of integrated circuits operating in a radiation environment, in general, and more particularly, to such storage cells that are tolerant of an upset fault, momentary or otherwise, caused by a radiation particle.
In radiation environments, radiation particles, like high energy particles and protons, for example, may cause logic circuits to incur upset faults under the same or different mechanisms. This problem of radiation logic upset is particularly severe in circuits utilized in spacecraft and high flying aircraft which operate in environments lacking in atmosphere. In these environments, there is substantially no atmosphere or very little atmosphere to absorb the radiation particles and thus, a higher density of particles may make contact with the logic circuits on-board such craft.
In addition, the problem is made even more critical by the substantial reduction in the integrated circuit geometries of the logic circuits which are currently approaching less than 0.5 microns, for example. When the geometries were greater than one micron, for example, the likelihood of a radiation particle logic upset was less severe. But, as these geometries shrank into the sub-micron dimensions, the likelihood of a radiation particular logic upset became greater. With geometries of less than 0.5 microns, this problem has become of paramount importance.
Currently, in storage cells of integrated circuits operating in a radiation environment, a triple voted circuit is used to mitigate the risks of a radiation upset causing a fault in the stored signal thereof. A triple voted circuit may be comprised of three latching registers configured in parallel with the data input signal of the storage cell coupled to the data inputs thereof. The three latching circuits are operated synchronously by a common clock signal to capture and store the same time sample of the input data signal. The data outputs of the latching registers may be coupled to a majority vote circuit which generates the stored signal of the storage cell. If any one of the latching registers incurs a radiation particle upset, it will be out-voted by the other two which produce the same and correct output. Accordingly, the storage cell output remains substantially unaffected by any single event upset to one of the storage registers thereof.
However, it is recognized that the radiation particle upsets are not limited to latching registers but may extend to the combinatorial logic surrounding the latching registers of a storage cell as well. Although radiation upsets of combinatorial logic circuits are momentary in nature, normally having a duration inversely proportional to the bandwidth of the circuit, for example, they may cause a corresponding momentary state change in the input data signal which is coupled to the data inputs of the three latching registers. Since the three latching registers are latched at the same time, it is possible to latch at the time of the momentary state change of the input data signal in which case, all three outputs of the latching registers will be in error. Thus, a majority vote will not result in the correct output for the storage cell.
With the integrated circuit geometries becoming smaller and smaller into the sub-micron region resulting in a higher frequency of upsets and with the higher operating frequencies of integrated circuits made possible by the smaller processing geometries, the possibility of concurrent latching of a common momentary upset becomes proportionately greater. The present invention provides for an improved integrated circuit storage cell intended to overcome this drawback of the current triple voted storage cell for use on-board spacecraft, high flying aircraft, and the like, which use is susceptible to radiation environments.
A storage cell of an integrated circuit is operable in a radiation environment to capture and store at predetermined time intervals a time sample of a data input signal. A signal representative of the stored data sample for each time interval is generated at an output of said storage cell. In accordance with the present invention, the storage cell comprises at least three data capturing circuits, each having a data input coupled to the data input signal and operable to capture and store a time sample of the data input signal at each predetermined time interval, the stored data sample of each said circuit being generated correspondingly at an output thereof; means coupled to the outputs of said data capturing circuits for generating at the output of said storage cell a signal representative of a stored data sample selected from at least two of said circuit outputs; and means coupled to said data capturing circuits for causing each data capturing circuit to capture a different time sample of the input data signal from the other data capturing circuits over each predetermined time interval. In one embodiment, the data input signal is a time varying digital signal and each data capturing circuit comprises a latching circuit so that the storage cell may encounter an upset by a radiation particle without affecting substantially the state of the output of the storage cell.